The LHC, the technological challenge

17 June, 1994

Geneva, 17 June 1994. Physicists at CERN1 talk almost casually about recreating conditions that existed only 10-12 second - a millionth of a millionth of a second - after the 'Big Bang', when our Universe might have been no bigger than a pinhead! This is however exactly what the high energy proton-proton collisions in the Large Hadron Collider (LHC) will do. To build instruments capable of creating such extreme conditions and then analysing the results with extraordinary precision is a daunting challenge which demands advances in many highly complex technologies. The success of the LHC is directly linked to the ability of CERN's scientists, in close collaboration with industry, to push the limits of known technology way beyond today's frontiers.

The LHC ring will contain of over 1000 superconducting bending magnets, each 13 metres long. These magnets are among the most technologically challenging components of the machine. Superconductivity is a property that some materials acquire at very low temperatures, when their resistance to the passage of electrical current more or less disappears. Under these conditions, large currents can flow easily through superconductors of small cross-section. This means that compact magnets can be built and operated for much lower cost than conventional 'warm' magnets made with copper or aluminium conductor. The only energy consumption of a superconducting magnet is that needed to refrigerate the conductor so that it remains superconducting.

LHC bending magnets have twin apertures through which two vacuum chambers will be threaded to contain the circulating proton beams. This unique and novel CERN design allows the counter-rotating beams to be housed in a single cryogenic magnet assembly. This results in a 25% lower cost and much more efficient use of the limited space in the LEP tunnel than if two completely separate rings of magnets were built.

For LHC protons to reach their collision energy of 14 TeV, the high technology superconducting electromagnets have to sustain a field of 8.65 Tesla, the highest ever used in an accelerator. To achieve this, the cable windings must be cooled to a temperature of 1.8K (-271.2û C), colder than outer space (2.7K). A major milestone in magnet development was passed when the first prototype LHC bending magnet was successfully tested at CERN on 14 April 1994. In the tests, the magnet, in its cryostat filled with superfluid helium, reached the LHC design field of 8.65 T at the first attempt without any "training", that is, the settling of the superconducting coils into the optimum position within the magnet. The first "quench" occurred at 8.67 T, when the coils lost their superconductivity because of hot-spots caused by micro-movements of the niobium/titanium conductors. The magnet was then powered again to 8.73 T and ran at this field level for about 15 min, without any further quench. This confirmed the validity of the magnet design and the ability of industry to take up the challenge of constructing the LHC.

To study the collisions of the tiny quarks locked deep inside protons requires a microscope on a larger scale than ever before built. But the microscope alone - LHC's 27-kilometre ring of superconducting magnets - is not enough. Researchers using it have to have sharp eyesight. Their 'eyes' are two mighty detectors, called ATLAS and CMS, each as high as a five-storey building, built like a Russian doll, with one module fitting snugly inside the other around the beam collision point at the centre. Each module, packed with state-of-the-art technology, is custom-built to do a special observation job before the particles fly outwards to the next layer. The interesting reactions when the hard quark grains in LHC's colliding protons clash head-on are extremely rare. Most of the time they graze past each other with little disturbance, providing less interesting physics.

To see enough interesting hard quark collisions, the physicists have to push for very high proton-proton collision rates. Collision rates are measured by what is called luminosity - the luminosity of a two-beam collider is the number of particles per second in one beam multiplied by the number of collisions per unit area in the other beam at the crossing point. For LHC, luminosities of a few times 1034 are needed, a hundred times higher than in any existing experiment. To achieve this, LHC's proton bunches, strung like beads on a chain 25 nanoseconds - 25 thousand millionths of a second - apart, will sweep through each other some 40 million times per second, each time producing about 20 interactions of one kind or another. Only one in a billion of these will be a hard quark collision, and the detector's data capture system has to select and filter out this event and get to work on it rapidly, so as not to miss the next one. Searching for a needle in a haystack seems easy in comparison.>

In addition, the wear and tear caused by so many high energy protons means that detectors have to be tough and durable, with precision components continuing to function year after year with minimal maintenance. To satisfy these stringent requirements, in 1990 CERN set up a Detector Research and Development Committee to oversee new projects. As a result of this work, several promising new technologies have been refined to a stage where they can now be used in LHC detector components.

The technological challenges of the LHC demands breaking new ground in superconductivity, high-speed electronics, cryogenics, super-computing, vacuum technology, material science and many other disciplines. The new technologies developed for LHC will become the fertile ground in which seeds for new hi-tech industries can flourish.

1. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, the Russian Federation, Turkey, Yugoslavia (status suspended after UN embargo, June 1992), the European Commission and Unesco have observer status.